The present invention is related to a power supply unit, more specifically to a power supply unit that is provided with a main battery and an auxiliary battery which supplies a voltage lower than the main battery and allows both batteries to be charged by a system power supply.
A hybrid car that is provided with an engine (internal combustion engine) has been applied to a practical use. In the hybrid car, drive wheels are driven by a motor at starting or in a lower speed range and the drive wheels are driven by the engine in middle and higher speed ranges to reduce fuel consumption and exhaust gas. Recently, a plug-in hybrid car that is provided with a battery that can be charged by a system power supply or a domestic power supply has been proposed to further reduce environmental load. For example, when a motor is driven by a battery that is charged with midnight power, a car can run a longer distance in an electric vehicle mode and the ratio of using electric power is increased compared to the ratio of using gasoline or other fuels. Therefore, compared to a general hybrid car, it is expected to reduce an emission of carbon dioxide and prevent air pollution. Since the system power supply reduces cost compared to supplying electric power separately, a fuel cost can be reduced if a battery is charged with low-cost midnight power.
Japanese Laid-Open Patent Publication No. 8-317508 discloses a charging device provided with a main battery and auxiliary battery that are charged by an external AC power supply. In the charging device disclosed in the publication, when the charging device is disconnected from the AC power supply, the auxiliary battery can be charged with electric power that is charged to the main battery. The charging device is provided with a rectifier, a reactor for smoothing, a switching circuit, and a transformer. The rectifier rectifies the AC power supplied from the AC power supply. The reactor is connected to an output terminal of the rectifier. The switching circuit controls output of the DC power supplied from the rectifier via the reactor. The transformer has a primary winding that is connected to the switching circuit, a secondary winding for the main battery, and a plurarity of secondary wingings for the main battery and the auxiliary battery. The charging device has an element that is connected to the secondary winding for the main battery of the transformer and allows electricity to flow bidirectionally. The charging device further has a switching rectifier circuit and another rectifier circuit. The switching rectifier circuit rectifies the AC voltage induced in the secondary winding and switches an output of the DC voltage supplied from the main battery. The another rectifier circuit is connected to the secondary winding for the auxiliary battery of the transformer and rectifies the AC voltage induced in the secondary winding. Further, the charging device has a control circuit that controls a switching operation of the switching circuit and the switching rectifier circuit.
When various electric appliances are used in a vehicle or outdoors, it has been conventionally required that DC power supplied from the battery of the vehicle be converted to AC power of 100 V by a DC/AC inverter.
In the charging device of the above publication, the AC power supplied from an external AC power supply is rectified by the rectifier, and thereafter supplied to a primary side of the transformer via the switching circuit and charged to the main battery and the auxiliary battery. The power that has been charged to the main battery can be charged to the auxiliary battery via the transformer.
Recently, it has been considered to be important that the electric appliances can be driven by the battery of a vehicle in case of emergency. However, since the main battery and the auxiliary battery of the charging device function as DC power supplies, they do not supply AC power for driving electric appliances. Therefore, it is required to provide a DC/AC inverter separately to use the main battery as a power supply for electric appliances. This increases the size of the device and makes it difficult to obtain a mounting space of the device.